Some epoxies work better on plastics than others. Modern plastics can be tricky to glue. But there’s more to repairing plastics than just making sure you bought the best epoxy for plastic. Prep work is essential too. While some epoxies definitely work better than others, the prep work can also make or break your project.
How to prepare your plastic for epoxy repair
Plastic is difficult to glue, because it’s inherently slippery. Contaminants from handling and from previous attempts at repair make it worse, because they interfere with the epoxy sticking. You need the epoxy to stick to the plastic, not the oils or the old adhesives.
If you’ve tried to glue the plastic before, make sure you scrape or sand off the old glue. If you’ve used super glue in the past and it failed, put the broken pieces in the freezer to make the old glue easier to remove.
If you’ve taped the pieces in the past, clean the pieces off with alcohol or, better yet, mineral spirits. Both are good at removing oils and other residue, but mineral spirits does a better job of removing old adhesives. Either will work, it’s just a matter of how much effort is required.
Sanding the two surfaces that you will be joining helps the epoxy to grip by giving it more surface area. It doesn’t take a lot of sanding, just rough the joining surfaces up a bit. Epoxy has good gap-filling properties, so this step takes advantage of that.
Best epoxy for plastic
Today there are lots of different specialty epoxies on the market. An epoxy formulated for metal may struggle on plastic, and an epoxy formulated for plastic probably won’t be ideal for metal. Confusingly, JB Weld was originally a brand of epoxy for metal. Today, kind of like WD-40, JB Weld is a brand, not a single product.
The go-to epoxy for plastic used to be Devcon Plastic Welder. It’s an excellent product, but all of the stores near me switched to JB Weld Plastic Bonder. Theoretically, JB Weld Plastic Bonder is slightly stronger, but we’re talking a difference of less than 10 percent.
How my last repair with epoxy and plastic went
My wife got a good deal on a used sewing machine because its case was cracked, and someone had simply duct taped it back together. That’s on brand for south St. Louis. Gluing a sewing machine enclosure back together isn’t very different from gluing a vintage computer case back together like I did for my Tandy 1000, so I told her I could fix it.
I couldn’t exactly tell what kind of plastic the case was made from. It definitely wasn’t plain old styrene, and I couldn’t quite tell if it was ABS. There are solvents for both styrene and ABS that will literally weld plastic pieces back together if they fit together tightly. A solvent-based plastic welder will be sligthly better if it works. Since the pieces didn’t all fit together perfectly and I wasn’t sure what kind of plastic it was, plastic epoxy was the safest bet.
I cleaned the joining surfaces with mineral spirits, sanded them lightly, then cleaned off the plastic dust with mineral spirits again. Then I mixed up the epoxy. Don’t mix up the whole tube of course, but always mix up a little more than you think you need. If you try to squeeze out just a couple of square millimeters of it, it’s much more difficult to get the correct ratio of resin and hardener. You’ll waste some this way, but you’ll waste less than you would if you have to do the repair again.
Preparing and using the epoxy
Squeeze out a somewhat generous amount of epoxy and hardener from the tube onto a disposable surface, then stir it with a wooden stick. Frequently the tube comes with a shortened popscicle stick for this purpose. Mix it until the color is consistent all throughout the mixture. Once you mix the two parts, you’ll have about a five-minute working time before it starts to set up.
Apply the epoxy to the smaller of the two pieces, then press it into place. If you get squeeze out, you can scrape it off the front with a popscicle stick or toothpick.
Once I had the pieces glued together, I used the stick to pull up as much of the excess as I could and I glommed it onto the inside of the surface to help reinforce it. It doesn’t look pretty, but it’s inside the case. And this joins the repair on two sides instead of just one.
The epoxy sets up in a few minutes, but needs a few hours to cure to full strength. I like to give it overnight. If you didn’t get all of the squeeze out, you can sand it flush with the surface to get it smooth. If the color of the epoxy doesn’t match your plastic, you can paint it to match.